The present invention relates to epitaxial layer structures of III-V compound semiconductors and more particularly to a thin film crystal having a planar doping structure and the method of growing the doped epitaxial layer structure.
As a high speed field-effect transistor (FET) based on III-V compound semiconductors, so called delta-doped, or planar-doped, FET is known. In IEEE Transaction on Electron Devices, Vol. ED-33, No. 5, May 1986, pp. 625-632, only a selected layer of GaAS is doped with silicon (Si) as an n-type impurity element while the GaAs epitaxial layer is grown by molecular beam epitaxy (MBE). Such delta-doped FET has a Dirac-delta-function-like doping profile, and has advantages of high gate-breakdown voltage and the high transconductance.
However, it is difficult to achieve an accurate control of the thickness of the epitaxial layer. Accordingly, it is extremely difficult to define the doping position in the order of atomic layer thickness at an arbitrary depth and thus its reproducibility is poor. Although Si can become the n-type impurity by occupying the Ga lattice points in the GaAs crystal, bonding strength between Si and As surrounding Si is not so strong when the crystal growth is carried out by the MBE technique, and thus high doping density is difficult to achieve with good reproducibility. Furthermore, the MBE growth tends to cause surface defects which have caused problems for high-speed LSI application.